1. Field of the Invention
This invention relates to a focus detecting device in a single lens reflex camera.
2. Description of the Prior Art
Already known is a focus detecting device for a single lens reflex camera in which a first image and a second image are formed by a pair of re-imaging lenses from a pair of light beams transmitted through two different areas of the exit pupil of a phototaking objective lens and the focused condition of the objective lens is detected from a variation in position of the first image and the second image relative to a pair of image position detecting photoelectric converters disposed on or near the focal planes of the pair of re-imaging lenses.
In order that such a focus detecting device may be compactly incorporated particularly at the bottom of the mirror box within the camera body, there is a problem to be solved because there are spatial limitations as compared with the incorporation of the device into the viewfinder. That is, to realize the focus detection based on the above-described principle, between the pre-imaging plane of the objective lens and the photoelectric converters, there must be a sufficient distance to enable the re-imaging lenses to re-image the image formed by the objective lens and this leads to the necessity of enlarging the dimensions of a part or the whole of the camera body.
Further, in incorporating such a focus detecting device into a camera, a field lens, for imaging the pupils of the re-imaging lenses near the exit pupil of the objective lens so that the image is contained within the exit pupil, is disposed near the pre-imaging plane of the objective lens, whereby the occurrence of any error in focus detection can be prevented. However, the field lens disposed at such a location comes close to an effective light beam passing from the objective lens to the film surface during photography, and this has led to the disadvantage that the field lens may interfere with part of the effective light beam or that the reflected light from the surface of the field lens may cause undesirable flare or ghosts on the phototaking picture plane.